Sorry, but didn't you give the number in your post ('Main number')?

The company is called Timberland Flooring Services Ltd.

Their fax is 01491 832 929 if that's any good.

A check of the Internet Archive gives the geographical number you've quoted and the above fax for contact.

Will they put you through on the geographical during office hours or do they refer you to the 0870?

This page on their site is about after care and gives the number 01491 821866.

A Google of "OX11 9BE" 01491 throws up this result which shows this company:
Timber Floor Supplies Ltd
North Farm
Wallingford Road
North Moreton, Didcot
Oxfordshire
OX11 9BE

Tel:  01491 835100
Fax: 01491 832929

This is the same fax number, so are they the same company?

The Internet Archive reveals that they have been using an 0870 number for only a couple of years. The current one seems to have been in use for about a year. Before that, they used 0870 7200988 for sales.

From the numbers I've found, none have a common prefix, so I can't see any way of knowing which block of DDI numbers to try.